The present invention relates to a bipolar transistor that employs a heterojunction as an emitter-base junction, and is concerned with semiconductor devices whose current gain is high and independent of the emitter area and which exhibit high-speed characteristics.
Conventional heterojunction bipolar transistors suffer from the problem that the current gain is strongly dependent on the emitter area and, as the area decreases, the gain lowers by a large margin. This problem is considered attributable to the surface recombination of carriers injected into the base that takes place at the periphery of the emitter-base junction. In order to overcome the above-described problem, it has heretofore been conventional practice to change gradually the Al fraction in the base layer from the emitter toward the collector so as to slope the energy band gap and thereby form a built-in field as in the case of the GaAlAs-GaAs heterojunction transistor disclosed in Japanese Patent Laid-Open No. 61-201466 (1986) by way of example.
The above-described prior art enables an improvement in the emitter area dependency but still suffers from the problem that the fraction must be accurately controlled when the base layer is produced and this may hinder the achievement of high reproducibility and productivity.